helmutforren
Newbie
- Joined
- Feb 20, 2017
- Messages
- 3
(((Hopefully I found an acceptable forum for posting this question. There are so many...)))
BACKGROUND:
I have an existing commercial product that uses a TEKK brand transmitter, in combination with custom POCSAG encoding hardware and software, to send a page to a paper receiver. This has worked great for over a decade.
Now the TEKK brand transmitter is being discontinued. I am having difficulty getting an alternate transmitter to work properly. Since the TEKK transmitter was costing us under $100 each, I'm trying to find something that's at least under $150, as opposed to one I found that works but costs $700.
Note that I have over 40 years electronics experience, but relatively little radio experience.
PROBLEM:
I've had partial success with a Friendcom (Chinese, of course) FC-302 transmitter. But I am not getting any customer support to help me get all the way to full success, even with the promise of volume purchases.
When I try to send a page through the FC-302, the pager receiver most often receives the page, but after 10 or 20 out of 50 chars, the page gets corrupted and truncated. Sometimes, the pager receiver doesn't get the page at all, presumably due to corruption within the CapCode portion of the page. Sometimes, the pager simply says "tone only", presumably due to some other kind of corruption.
This is definitely a problem with the transmitter, and most likely my method of driving it. But it's NOT a problem upstream, with my software or hardware specifically. That is, that I can get my software and hardware running, connect the old TEKK transmitter, and get great perfectly formed pages on the receiver. Then I can unplug the TEKK and plug in the FC-302, and start getting corrupt pages. The 12V power supply level, the PTT signal, and the data signal all look good on the scope.
So I need help figuring out why the FC-302 pages are partially OK but not totally OK. Chinese support hasn't helped, so perhaps one of you can help.
Note that I put a spectrum analyzer nearby. The trace on it looks very similar between the two. I can't see any difference. It's a circa 1996 spectrum analyzer (BK Precision 2630) that seems to have at best a 20kHz bandwidth and 100kHz/division display. There might be a difference in the two radio signals that's just too small for me to see with this tool.
Note also that I can put the spectrum analyzer in "scan mode". I can only guess what that does. I guess that it shows me a signal across time, for demodulating at the knob-tuned frequency. When I do this, I see a square wave that's a facsimile of my input data signal. So I believe this is in essence doing the signal demodulation to give me a real time signal, just like the pager receiver does. Well, these look very similar between the TEKK transmitting and the FC-302 transmitting. However, the vertical position on the screen depends a lot on the center frequency chosen by knob (and drifting). The peak-to-peak magnitude of the signal is never more than half a vertical division (no scale provided by tool, that I can see), for both transmitters. So, again, I can't see a difference with this tool.
What else might I try? What else might be my problem? Surely, the FC-302 signal is close enough that the pager receiver *almost* gets it. But not quite. There ought to be some other thing I can do to bring this home. Any ideas?
Note some more things. I don't know the spec for frequency shift that I *should* have for POCSAG. I don't know the shift created by the working TEKK. I don't know the shift created by the not-quite-working FC-302. I don't think I can see this with my spectrum analyzer. I don't know if I need to rent another tool to check this, or if I can get the problem solved without a new tool, just some expert advice instead. The FC-302 should, I think, be causing a shift of 2.5kHz to 3.5kHz.
I have Friendcom's "FSK" modem inside the FC-302, but I think I'm bypassing it anyway. They also have a "4FSK" and "GMSK" modem. However, since I'm encoding the POCSAG myself and simply driving a data line, I don't think I'm using the modem at all. I don't know if I can twiddle my voltage input levels to get more frequency shift. I'm currently providing contact closure to ground again the radio's 5V pull-up.
MEANWHILE, I've tried a much more expensive transmitter from Apollo, their TX125EN. It works flawlessly, but costs too much. On my spectrum analyzer, however, rather than seeing just a peak, I get a rather wide flat top. So this tells me the TX125EN is outputting a much, much wider band signal than the TEKK or FC-302. The TEKK works with the smaller bandwidth. Maybe the FC-302 is much smaller, but I can't see it with my tools. Am I grasping at straws now???
Any and all advice is greatly appreciated.
Oh, one more thing. As described, I'm encoding the POCSAG myself, and proven successful by the working TEKK transmitter for the last decade. I could instead actually let the Friendcom FSK modem do that POCSAG encoding instead. But when I try to talk directly to the FSK modem over RS232, I get no response whatsoever. Friendcom provides zero doc about the RS232 protocol. I tried "AT<CR>", but get nothing. I also have no clue what baudrate. I've tried 1200 and 9600 and 19200, perhaps even 115,200. I tried a message as used with the TX125EN, that begins with <STX> and ends with <EOT>, but I get no response or radio frequency out of the FC-302. If only they documented the protocol on their RS232 input. Any guesses?
Thanks VERY much,
Helmut
BACKGROUND:
I have an existing commercial product that uses a TEKK brand transmitter, in combination with custom POCSAG encoding hardware and software, to send a page to a paper receiver. This has worked great for over a decade.
Now the TEKK brand transmitter is being discontinued. I am having difficulty getting an alternate transmitter to work properly. Since the TEKK transmitter was costing us under $100 each, I'm trying to find something that's at least under $150, as opposed to one I found that works but costs $700.
Note that I have over 40 years electronics experience, but relatively little radio experience.
PROBLEM:
I've had partial success with a Friendcom (Chinese, of course) FC-302 transmitter. But I am not getting any customer support to help me get all the way to full success, even with the promise of volume purchases.
When I try to send a page through the FC-302, the pager receiver most often receives the page, but after 10 or 20 out of 50 chars, the page gets corrupted and truncated. Sometimes, the pager receiver doesn't get the page at all, presumably due to corruption within the CapCode portion of the page. Sometimes, the pager simply says "tone only", presumably due to some other kind of corruption.
This is definitely a problem with the transmitter, and most likely my method of driving it. But it's NOT a problem upstream, with my software or hardware specifically. That is, that I can get my software and hardware running, connect the old TEKK transmitter, and get great perfectly formed pages on the receiver. Then I can unplug the TEKK and plug in the FC-302, and start getting corrupt pages. The 12V power supply level, the PTT signal, and the data signal all look good on the scope.
So I need help figuring out why the FC-302 pages are partially OK but not totally OK. Chinese support hasn't helped, so perhaps one of you can help.
Note that I put a spectrum analyzer nearby. The trace on it looks very similar between the two. I can't see any difference. It's a circa 1996 spectrum analyzer (BK Precision 2630) that seems to have at best a 20kHz bandwidth and 100kHz/division display. There might be a difference in the two radio signals that's just too small for me to see with this tool.
Note also that I can put the spectrum analyzer in "scan mode". I can only guess what that does. I guess that it shows me a signal across time, for demodulating at the knob-tuned frequency. When I do this, I see a square wave that's a facsimile of my input data signal. So I believe this is in essence doing the signal demodulation to give me a real time signal, just like the pager receiver does. Well, these look very similar between the TEKK transmitting and the FC-302 transmitting. However, the vertical position on the screen depends a lot on the center frequency chosen by knob (and drifting). The peak-to-peak magnitude of the signal is never more than half a vertical division (no scale provided by tool, that I can see), for both transmitters. So, again, I can't see a difference with this tool.
What else might I try? What else might be my problem? Surely, the FC-302 signal is close enough that the pager receiver *almost* gets it. But not quite. There ought to be some other thing I can do to bring this home. Any ideas?
Note some more things. I don't know the spec for frequency shift that I *should* have for POCSAG. I don't know the shift created by the working TEKK. I don't know the shift created by the not-quite-working FC-302. I don't think I can see this with my spectrum analyzer. I don't know if I need to rent another tool to check this, or if I can get the problem solved without a new tool, just some expert advice instead. The FC-302 should, I think, be causing a shift of 2.5kHz to 3.5kHz.
I have Friendcom's "FSK" modem inside the FC-302, but I think I'm bypassing it anyway. They also have a "4FSK" and "GMSK" modem. However, since I'm encoding the POCSAG myself and simply driving a data line, I don't think I'm using the modem at all. I don't know if I can twiddle my voltage input levels to get more frequency shift. I'm currently providing contact closure to ground again the radio's 5V pull-up.
MEANWHILE, I've tried a much more expensive transmitter from Apollo, their TX125EN. It works flawlessly, but costs too much. On my spectrum analyzer, however, rather than seeing just a peak, I get a rather wide flat top. So this tells me the TX125EN is outputting a much, much wider band signal than the TEKK or FC-302. The TEKK works with the smaller bandwidth. Maybe the FC-302 is much smaller, but I can't see it with my tools. Am I grasping at straws now???
Any and all advice is greatly appreciated.
Oh, one more thing. As described, I'm encoding the POCSAG myself, and proven successful by the working TEKK transmitter for the last decade. I could instead actually let the Friendcom FSK modem do that POCSAG encoding instead. But when I try to talk directly to the FSK modem over RS232, I get no response whatsoever. Friendcom provides zero doc about the RS232 protocol. I tried "AT<CR>", but get nothing. I also have no clue what baudrate. I've tried 1200 and 9600 and 19200, perhaps even 115,200. I tried a message as used with the TX125EN, that begins with <STX> and ends with <EOT>, but I get no response or radio frequency out of the FC-302. If only they documented the protocol on their RS232 input. Any guesses?
Thanks VERY much,
Helmut